Question

What happens when a sound source is moving?

Answer 1

Explanation:

It stretched or compresses the sound waves it is producing if it is relatively moving away to getting closer respectively. This is called the Doppler shift effect..

This is why when a wailing ambulance approached you, the pitch of its wailing is high but when it passes you and moves away from you the pitch of the wailing lowers. Remember this is happening relative to your position.